Passive needs changing. At the moment, the range is only useful for encounters with melees, but as soon as someone comes within melee he becomes insane strong. My suggestion:
By the Shield(Innate): Cain gains 4/5/6 armor and magic resistance for each allied champion within 800 range and grants half of this amount to such allies.
Reasons:
500 Range is too small. Only helps when really close to a teammate, REALLY, so early game strength was extremely weak, very situational.
To counteract the range enhancement, which turned the passive from almost never to standard, the values needed to be lowered a tad.
Granting the bonus to all allies seemed to strong, so I decided that it should probably only be to the allies effecting it.
For those that think the armour levels are too low late game, think about it this way. In a team fight you would probably have about 2 allies in the range. The old passive would grant 24 for you, and 12 for the rest. Now, it would be easier to have all 4 allies in range. 24 for you, 12 for the rest.
The maximum for the old one was 48 bonus armour, which I thought was too much. If Cain built tank it would barely matter for him, but his squishy allies would get 24 armour, which is quite massive.